There are many new technologies being developed and existing technologies being refined to meet ever more stringent automotive exhaust emission standards. The two general areas of development for reducing automotive exhaust emissions are: (1) reducing engine generated exhaust emissions and (2) optimizing after-treatment of engine generated exhaust emissions. Exhaust after-treatment generally involves one or more catalytic converters in the engine exhaust path.
One area where there is significant room for improvement is the reduction of emissions during the cold start period, i.e. the period of time from when the engine is started to when catalyst light-off occurs. During the cold start period, generated exhaust hydrocarbon (HC) emissions are high and the performance of the exhaust after-treatment system is low. The cold start period lasts approximately 30-60 seconds with current (OEM) emission control strategies and accounts for up to 80% of the total measured tail pipe HC exhaust emissions under the federal test procedure FTP75 used for evaluating the emission of automotive engine systems.
There are three primary engine operating parameters that are generally controlled for optimizing engine performance; air, fuel, i.e. air-fuel (A/F) ratio, and ignition timing. Similarly, there are three primary engine operating parameters that need to be controlled for optimizing the conversion efficiency of a three-way catalyst; air, fuel, i.e. A/F ratio, and catalyst energy (temperature). Any emission control strategy involving control of engine A/F ratio, and ignition must be performed within the limits of satisfactory driveablilty, thus limiting the range of control options.
Automotive tail pipe emissions are conventionally minimized by closed loop control of engine air and fuel by way of feedback from an oxygen sensor mounted in the engine exhaust path. The oxygen sensor measures the excess oxygen in the engine exhaust gas and the resulting sensor output signal is used to correct the engine fuel injection period for each cylinder event. Based on the engine A/F ratio (computed from the oxygen sensor output signal) the exhaust gas concentrations (directly oxygen and empirically HC, CO, NOx) that enter the catalyst from the engine exhaust are computed. There are additionally, limited computer models that predict catalyst energy (temperature) as a function of A/F ratio and exhaust gas temperature. Thus, when the oxygen sensor is active, overall optimization of the emission control process may be performed.
However, emission control based primarily upon sensing excess oxygen in the engine exhaust has limitations. First, there is no feedback from the oxygen sensor during the cold start period since the oxygen sensor is not yet active. Second, there are, currently, no robust exhaust gas temperature models for cold catalyst operation. Available catalyst energy models are limited to hot, stabilized, steady state conditions and an assumed catalyst aging condition. Thus, emission control is effectively open loop and based on many improper assumptions under cold start conditions. Further, lacking any direct measurement of catalyst energy, overall emission control is sub-optimum since catalyst energy models are accurate only under very limited conditions.
It is well understood that total hydrocarbon emissions are reduced with more rapid catalyst light-off. However, increasing catalyst heating by adjusting A/F ratio, ignition timing etc. generally results in higher exhaust emission rates. Stoichiometric A/F ratio control is more optimum for minimizing exhaust emission rate and is not optimum for maximum catalyst heating. Minimizing total emissions requires shifting control strategies from maximum catalyst heating to stoichiometric control when catalyst light-off occurs.
Directly measuring catalyst temperature and using the catalyst temperature as an additional engine control variable allows one to safely adjust the engine operating parameters to achieve a more aggressive catalyst heating function during the cold start period than is possible under a strictly open loop emission control strategy. Experimental data shows that the total tailpipe emissions can be reduced using a strategy of maximum heating of the catalyst during the cold start period compared to strictly minimizing the rate of generated engine exhaust gas HC emissions.
In addition to minimizing undesired exhaust emissions, more modem automotive emission control systems are required to monitor tailpipe HC emissions and to actuate a malfunction indicator in the vehicle if a threshold value of undesired emissions is exceeded. More modem emission control systems are also required to isolate the cause of excessive emissions to a malfunctioning component.
The dominant cause of increased tail pipe emissions at low emission vehicle (LEV) and ultra low emission vehicle (ULEV) levels is degraded catalyst light-off performance. A current approach for diagnosing catalytic converter performance is the use of pre-catalyst and post-catalyst oxygen sensors to determine the warmed up oxygen storage capacity of the catalytic converter. However, the measurement of oxygen storage capacity does not provide reliable information regarding the critical light-off performance of the catalyst. Other approaches have sought to measure the exothermic activity in the catalytic converter by monitoring the difference between the catalytic converter inlet and outlet temperatures. These approaches require multiple temperature sensors and require interpretation of small temperature differences under widely varying conditions to discriminate malfunctioning converters from properly functioning converters.
Satisfactorily functioning catalysts exhibit an increase in the rate of temperature rise following catalyst light-off due to a release of exotherm energy from the catalyst. FIG. 1 illustrates a typical catalyst temperature profile during a light-off period. Experiments have indicated that by adjusting engine operating parameters to heat the catalyst at a constant rate during the cold start period, it is possible to determine degraded catalyst HC conversion by measuring the time that it takes for the catalyst to achieve a predetermined target temperature. Constant rate heating of the catalyst is consistent with maximum heating of the catalyst prior to catalyst light-off. The present invention utilizes an Energy Control and Optimization (ECO) strategy of constant rate maximum catalyst heating during the cold start period combined with the use of catalyst temperature as an additional emission control variable to provide for the simultaneous reduction of cumulative tail pipe HC emissions and for the accurate determination of catalyst performance using only a single temperature sensor for measuring the temperature of the catalyst in the catalytic converter.